The invention relates to a field-effect transistor. [1] has disclosed a wide range of different field-effect transistors. One example of a field-effect transistor of this type is what is known as the MOS field-effect transistor.
According to current technology, a MOS field-effect transistor still has a chip surface area of at least approximately 0.8 xcexcm2 to 1.5 xcexcm2.
Furthermore, basic principles of what are known as carbon nanotubes are known from [2] and [4].
A process for producing carbon nanotubes by growing them on a substrate is known from [3].
A further process for producing carbon nanotubes by vapor deposition of the carbon nanotubes is described in [4].
Furthermore, [5] has disclosed a process in which an electrically semiconducting carbon nanotube or metallically conductive carbon nanotube is converted into a boron nitride nanotube, which has an electrically insulating action, by means of doping with boron atoms and nitrogen atoms.
Furthermore [6] has disclosed a field-effect transistor with a carbon nanotube which couples two gold electrodes to one another via silicon dioxide substrate in such a manner that they can be electrically controlled. In this case, the gold electrodes form the source region and the drain region of the field-effect transistor, and the controlled channel region of the field-effect transistor is formed by the carbon nanotube. The electrical properties, in particular the electrical conductivity, of the carbon nanotube which forms the channel region is controlled by means of a silicon layer which is located below the silicon dioxide layer and is used as gate region of the field-effect transistor.
Furthermore, a process for producing a silicon nanowire is known from [7]. Therefore, the invention is based on the problem of providing a field-effect transistor which takes up less space that the known field-effect transistors.
A field-effect transistor has a nanowire, which forms a source region, a channel region and a drain region of the field-effect transistor, the nanowire being a semiconducting and/or metallically conductive nanowire. Furthermore, the field-effect transistor has at least one nanotube, which forms a gate region of the field-effect transistor, the nanotube being a semiconducting and/or metallically conductive nanotube. The nanowire and the nanotube are arranged at a distance from one another in such a manner or set up in such a manner that it is substantially impossible for there to be a tunneling current between the nanowire and the nanotube, and that the conductivity of the channel region of the nanowire can be controlled by means of a field effect as a result of an electric voltage being applied to the nanotube.
According to one configuration of the invention, the field-effect transistor has, as nanowire, a first nanotube, which forms a source region, a channel region and a drain region of the field-effect transistor. The first nanotube is a semiconducting and/or metallically conductive nanotube. Furthermore, according to this configuration of the invention, the nanotube which forms the gate region is formed by a second nanotube, the second nanotube being a semiconducting and/or metallically conductive nanotube. The first nanotube and the second nanotube are arranged at a distance from one another which is such that it is substantially impossible for there to be a tunneling current between the nanotubes and that the conductivity of the channel region of the first nanotube can be controlled by means of a field effect as a result of an electric voltage being applied to the second nanotube.
Since the field-effect transistor is formed substantially from nanotubes, therefore, the result is a transistor which takes up a considerably smaller surface area than the known field-effect transistors.
Furthermore, a switching operation between two states of the field-effect transistor is possible with a greatly reduced power loss, in particular on account of the considerably lower capacitance compared to conventional field-effect transistors and the very good electrical conductivity in particular of the carbon nanotubes.
The conductivity of the first nanotube is varied as a result of local application of an electrical potential and therefore of an electric field in particular in the section of the first nanotube which forms the channel region, creating the functionality of a field-effect transistor.
Any material can be used as the material for the first nanotube or the nanowire, provided that the first nanotube or the nanowire has electrically semiconducting and/or metallically conductive properties.
Moreover, the source region or the drain region and the channel region of the first nanotube or of the nanowire may be doped. In this way, it is possible to produce a potential barrier in the channel region, leading to a reduction in leakage currents in the quiescent state. The source region and the drain region as well as the channel region are preferably doped in such a way that a pn junction or an pn junction is formed both between the source region and the channel region and between the drain region and the channel region.
However, it is also possible for only the source region or the drain region or the channel region to be doped. To reduce leakage currents, in the case of pn junctions or pn junctions, it is furthermore expedient to leave small areas with a size of approximately 1 nm to approximately 5 nm of the nanotube between the n-doped region and the n-doped region in undoped form.
Obviously, the invention can be considered to lie in the fact that a second nanotube is arranged in the vicinity of the channel region of the first nanotube or of the nanowire, as a controlling element, in such a manner that the conductivity of the first nanotube can be controlled as required in that part of the first nanotube which forms the channel region.
It should be noted, that according to a refinement of the invention, the nanowire and the nanotube which forms the gate region or the two nanotubes do not touch one another, i.e. they are not brought into physical contact with one another, but rather are separated from one another by a dielectric, in the most simple case by air, a gas or vacuum. Nevertheless, it should be ensured that the conductivity of the first nanotube can be influenced to a sufficient extent by means of the field effect.
Alternatively, the dielectric may also be formed by an electrically nonconductive gas which is introduced between the two nanotubes.
The shortest distance between the first nanotube and the second, controlling nanotube is selected as a function of a maximum tolerable tunneling current between the two nanotubes and the desired supply voltage with which the field-effect transistor is operated.
By way of example, in the case of two carbon nanotubes, with the second nanotube oriented substantially perpendicular to the first nanotube, given a diameter of the two nanotubes of from 1 nm to 10 nm, the distance is to be selected in a range from 0.5 nm to 5 nm. In this case, obviously, the two nanotubes are arranged in a T shape with respect to one another, so that the field-effect transistor has a T-shaped structure.
Furthermore, it is also possible for an insulator layer, i.e. a layer of electrically nonconductive material, for example of an oxide material, e.g. of silicon dioxide, or of a nitride material, e.g. of silicon nitride, to be used as the dielectric.
In this context, it is merely necessary for it to be substantially impossible for there to be a flow of current between the two nanotubes, or at most for a negligible tunneling current to be possible.
The nanotubes may be designed as semiconducting and/or metallically conductive carbon nanotubes. Furthermore, it is possible to use single-walled or multi-walled nanotubes, in particular carbon nanotubes.
The second nanotube may have three ends, in which case an electrical voltage can be applied to one end and the two further ends are arranged in such a manner that, on account of the applied electrical voltage, they can be used to change the conductivity of the channel region of the first nanotube.
This refinement makes it possible to increase the size of the channel region, i.e. the active region in which the conductivity can be changed, with the result that leakage currents which occur in the blocking state of the field-effect transistor are reduced considerably.
According to a further configuration of the invention, the field-effect transistor has two gates, each of which can be used to switch the field-effect transistor, i.e. to change the conductivity in that part of the first nanotube which forms the channel region of the field-effect transistor.
This configuration improves the susceptibility to errors and the resistance to interference.
In this case, there is a third nanotube, which forms a second gate region of the field-effect transistor, the third nanotube being a semiconducting and/or metallically conductive nanotube. The first nanotube and the third nanotube are arranged at a distance from one another which is such that it is impossible for there to be a tunneling current between the nanowire and the nanotubes and that the conductivity of the channel region of the nanowire or of the first nanotube can be controlled by means of a field effect as a result of an electric voltage being applied to the third nanotube.
Furthermore, the second nanotube and the third nanotube may be electrically coupled to one another.
In general terms, it is in principle possible for a predeterminable number of further carbon nanotubes to be grown on further insulator layers as further gates of the field-effect transistor, with the result that a simple logic OR arrangement is created by means of a field-effect transistor.
Furthermore, according to a further configuration of the invention each nanotube may have a plurality or even a multiplicity, i.e. an entire bundle, of individual nanotubes, with the result that the stability and reliability of the field-effect transistor which is formed are further improved.
The ends of the nanotubes used in the field-effect transistor may optionally be open or closed.
According to a further configuration of the invention, the field-effect transistor has a nanowire, which forms a source region, a channel region and a drain region of the field-effect transistor. A nanotube, which has an insulating region and a semiconducting region or a metallically conductive region, is applied to the nanowire. The insulating region of the nanotube is applied to the channel region of the nanowire, in such a manner that the insulating region of the nanotube forms an insulator of the field-effect transistor. Furthermore, the nanotube is applied to the nanowire in such a manner that the semiconducting region of the nanotube or the metallically conductive region of the nanotube forms a gate region of the field-effect transistor.
The nanowire may be a silicon nanowire.
According to an alternative configuration of the invention, the nanowire may be a further nanotube, for example a carbon nanotube. A particular advantage of this configuration is the compactness of the solution, i.e. the fact that both elements of the field-effect transistor, namely both the controlling element (gate) and the controlled element (channel), are each formed by a carbon nanotube, with the associated advantageous material properties.
The further carbon nanotube may have at least one semiconducting region and at least one metallically conductive region, it being possible for the semiconducting region to be arranged between two metallically conductive regions.
In this case, the semiconducting region of the further carbon nanotube preferably forms the channel region of the field-effect transistor, and the two metallically conductive regions form the source region and the drain region of the field-effect transistor.
The nanotube may be composed of a plurality of individual nanotubes, for example of an electrically insulating nanotube, according to one configuration of the invention a boron nitride nanotube, and one or more semiconducting or metallically conductive carbon nanotubes.
The insulating region of the nanotubes may be formed by a boron nitride nanotube.
In general terms, in this context it should be noted that carbon nanotubes have an electrical conductivity which is dependent on the tube parameters. For example, depending on the tube parameters there are electrically semiconducting carbon nanotubes and metallically conductive carbon nanotubes. By contrast, boron nitride nanotubes which are structurally identical are electrically insulating, since they have an energy band gap of 4 eV. The semiconducting region or the metallically conductive region of the nanotube may be a carbon nanotube, i.e. a semiconducting carbon nanotube or a metallically conductive carbon nanotube.
It is therefore possible to use both standard types of carbon nanotubes, with the result that the relatively complex checking of the electrical properties of the carbon nanotubes produced in each case could even be dispensed with if desired and, by way of example, the transistor parameters do not necessarily have to be known.
Within the context of the invention, it is possible to use both single-walled and multi-walled (carbon) nanotubes.
A further field-effect transistor has a nanowire, which forms a source region, a channel region and a drain region of the field-effect transistor. At least one electrically insulating nanotube, which forms an insulator of the field-effect transistor, is applied to the nanowire. Furthermore, the field-effect transistor has at least one electrically semiconducting or metallically conductive nanotube which is applied to the insulating nanotube and forms a gate region of the field-effect transistor.
Evidently, the electrically insulating nanotube or the insulating region of a nanotube is applied to the nanowire in such a manner that, by means of the semi-conducting or metallically conductive nanotube which forms the gate region, it is possible to control the density of the electrical charge carriers by means of a field effect which is established in the channel region of the field-effect transistor, i.e. in the nanowire, and therefore to switch the field-effect transistor between two transistor states, namely a conductive transistor state and a nonconductive transistor state.
Therefore, the channel conductivity of the field-effect transistor according to the invention, as in conventional field-effect transistors, is controlled by means of an electric field by an electric voltage which is applied to a gate electrode, i.e. by an electric voltage which is applied to the gate region of the field-effect transistor.
A considerable advantage of the invention is that the field-effect transistor which has been described is very small, i.e. its size may be produced with dimensions of as little as 100 nm2 and below.
Furthermore, the fact that a monomolecular, i.e. single-piece field-effect transistor is formed from nanotubes, generally from a nanowire and a nanotube, means that the electronic component can very easily be integrated in interconnects of an electric circuit.
Furthermore, a further advantage of the invention resides in the compatibility of the field-effect transistors with respect to silicon material which is usually employed in semiconductor circuits.
In general terms, the invention can be considered to lie in the fact that a further nanotube is applied to a nanowire, for example a carbon nanotube, in such a manner that the single-piece, i.e. monomolecular element which is thereby formed as the field-effect transistor represents a separate electronic, monomolecular component.